1. Field of the Invention
This invention relates to novel compositions and a process of their preparation and application for improving the resistance of fabricated metal components subjected to corrosive conditions, by lining or coating those components with highly inert melt-processable plastic material, more particularly a perfluoroalkoxy side chain modified fluorocarbon polymer composition especially suited to rotolining, rotomolding and even rotocoating processing. Although these processes differ technically in a number of respects, for the sake of convenience the term "rotomolding" is used herein to refer to all three generically unless otherwise indicated.
2. Description of the Prior Art
In chemical engineering, energy generation and similar industrial applications, there are frequent requirements for equipment such as pumps, tanks or other vessels and associated ducting for handling fluids under severe service conditions. Not only are the fluids themselves frequently corrosive to the metals with which they come in contact, but they may also be under substantial pressure and/or at elevated temperatures, thus imposing severe stresses on the equipment. In services involving such aggressive environmental conditions, the choice of metals or alloys suitable or capable of giving practical service life becomes very limited indeed, and even those metals which may be acceptable often are not available. Frequently the equipment designer is confronted with a dilemma in selecting between a material suitable to meet strength requirements necessitated for mechanical reasons, and a different material dictated by corrosion resistance considerations. Unfortunately a substantial compromise is generally involved if a single construction material is used. Accordingly resort is commonly made to combining two or more materials to satisfy the conflicting requirements.
The combination of plastic materials as linings, coatings or films with base metal components has long been practiced as a means of securing the strength advantage of metals concomitantly with the corrosion resistance of certain plastics in fabricating a final product. The use of plastic coatings and/or linings with metal components has accordingly found considerable application in industry, as illustrated by patents issued in this field. An example is found in U.S. Pat. No. 3,432,511 which discloses the use of a partially degraded polytetrafluoroethylene (PTFE) powder dispersed in a suitable liquid vehicle or carrier for brush or spray coating application. In U.S. Pat. Nos. 3,438,388, 3,459,213 and 3,537,700, again PTFE powder compositions are employed but in these cases are isostatically molded to form linings for conduit valves or the like.
The choice of PTFE polymers for corrosion resistance service is a natural one arising from their excellent physical properties and inertness to chemical attack. However PTFE polymers are not adapted to conventional thermal molding processes, since they lack the melt-processing capability of commonly used coating materials such as polyethylene, nylon, fluorinated ethylene propylene copolymers ("FEP"), "Halar" (ECTFE copolymer), etc., for example. Special molding procedures must therefore be resorted to. On the other hand those above-named readily melt-processable materials lack the same degree of high temperature resistance, lubricity and superior chemical inertness exhibited by PTFE polymers.